Photometry of Diffuse Reflexion of Light on Matt Surfaces. 199 



the wire, the correction amounting to about j 1 ^ per cent, in 

 the value of E. 





E. 



n. 



E/». 



cr. 



•258 

 •376 

 •378 

 •381 

 •598 

 •608 

 •614 

 •887 

 1-207 





1-981 X10 1 - 

 1-022 

 1-201 

 •778 

 1-240 

 1-292 

 2395 

 1-359 

 1-155 



7-872X10 11 



3-715 



4-359 



2-816 



3-880 



4018 



7-424 



3-602 



2-618 



2516 

 2-751 

 2-755 

 2-762 

 3-195 

 3-217 

 3-227 

 3-773 

 4-414 







Silver (bard-drawn) 



Copper (hardened by stretching). . 



Nickel (hardened by stretching)... 



German-silver (hard-drawn) 



For the last four substances E/n is greater than 3, and 

 consequently a is greater than J. If the material were iso- 

 tropic, this would imply that the bulk-modulus is negative, 

 or, in other words, that a hydrostatic pressure applied to the 

 material would result in an increase of bulk. The inference 

 is that wires of these four substances are so far from being 

 isotropic that it is improper to apply the theory of isotropic 

 solids to them. 



XIII. Photometry of the Diffuse Reflexion of Light on Matt 

 Surfaces. By Henry Robert Wright, Ph. D* 



Historical Introduction. 



LAMBERT'S f law (g = V 1 .ds. cos i cos e), which is known 

 under the name of the law of the cosine, has been 

 shown especially by Seeliger's i investigations not to be 

 rigidly true, but only approximately so in a very few cases. 

 No theoretical proof has so far been adduced and the 

 experiments made by Bouguer §, Kononowitsch||, Seeliger ; 



* Communicated by the Author ; published also in Wied. Ann. vierte 

 Folge, Bd. i. p. 17, 1900. 



t J. H. Lambert, ' Photometria sive de mensura et gradibus luminis 

 colorum et umbrae,' Augsburg, 1760. 



\ Seeliger, Vierteljahrsschr. der Astr. Gesellsch. xx. Heft 4, p. 267, 

 1885, and xxi. Heft 3, p. 216, 1886, and Sitzungsber. der math.-phys. 

 Classe der konigl. bayr. Acad. d. Wissensch. Heft 2, p. 201, 1888. 



§ Bouguer, ' Histoire de l'Academie Eoyale des sciences.' Paris, 1757 

 (1762), and ' Traite d'optique sur la gradation de la lumiere. Ouvrage 

 posthume de M. Bouguer, et publie par M, FAbbe de Lacaille' : Paris, 

 1760. 



|| A. Kononowitsch,£c/in/i!m der math. Abth. der Neurussischen natur- 

 forsch. Gesellsch. t. ii. ; Fortschr. d. Physik, xxxv. p. 430, 1879, and 

 Schriften der neuruss. (Kiew) Univ. xxii. 107 ; Fortschritte der Physik, 

 xxxvii. 2, p. 481, 1881. 



